In filtration, it is desirable to maximize filter area while minimizing the volume of the filter construction employed. One common means for attaining this result is to provide a construction formed by spirally winding a multi-layer material having a filter layer to form a tightly wound cylinder into which liquid is introduced at one spiral end and removed from the opposite spiral end. Means must be provided in the filter construction for assuring that all entering liquid passes through the filter medium prior to being removed, that is, the entering liquid must be prevented from simply passing through the spaces between the wound filter material without passing through the filter medium. It has been heretofore proposed to employ corrugated filter media in spiral wound filter constructions to maximize filter surface area for a given filter construction volume. In these constructions, sealing arrangements are provided at each spiral surface of the wound filter to assure that incoming fluid passes through a filter surface before leaving the filter unit. Such constructions are shown for example in U.S. Pat. Nos. 2,322,548 and 2,599,604 and 3,025,963. However, a corrugated spiral wound filter construction has undesirable strength characteristics which has limited its use. When fluid is introduced into the corrugated construction, even under moderate pressure or when filter loading is increased due to particle accumulation, the flutes become compressed along their entire length resulting in severe stress being placed upon the filter material. Under these compression forces, the chances of rupturing the filter material and rendering the construction useless are greatly increased. Due to these undesirable strength characteristics, practical utilization of filter units of this type i.e., with corrugated filter material, have had only limited usage. It would be highly desirable to provide a filter construction having a large filter area per unit volume and which permits the use of relatively high differential pressures across the filter material without a high risk of rupturing the filter medium.
A spiral-wound filter construction having such improved strength characteristics is described in an application entitled SPIRAL-WOUND FILTER filed Apr. 22, 1971, Ser. No. 136,315, now U.S. Pat. No. 3,722,696 in the names of James Dwyer and Peter Reiman and assigned to the assignee of this application. The filter is formed by spirally winding a multilayer web comprising at least one fluid-impermeable layer, at least one filter layer and at least two generally flat spacer layers having a high void volume. The spacer layers are permeable to edgewise fluid flow and are positioned adjacent each surface of the filter layer. They provide support for the filter material and a means for fluid to pass axially through the wound filter. The spacer layers are selectively sealed so that, in use, unfiltered fluid entering one spiral end surface must pass through at least one filter layer prior to leaving the opposing spiral end surface. In the construction disclosed in the foregoing application, the fluid-impermeable layer is employed to separate spacer layers sealed from the atmosphere on opposing spiral end surfaces, i.e., to separate a spacer layer one of whose edges is open on the upstream side from a spacer layer, one of whose edges is open on the downstream side.
Although this construction has great advantages over the prior art construction because of its greatly improved strength characteristics, it, like the prior art constructions, employs a fluid-impermeable layer to attain the desired fluid flow. It would be highly desirable to eliminate the fluid-impermeable layer since it is essentially a "non-working" layer in that it blocks fluid passage and does not filter incoming fluid. Thus, the fluid-impermeable layer increases the volume of a wound filter construction without increasing its filtration capacity. Accordingly, it would be highly desirable to provide a spirally-wound filter construction which eliminates the need for a fluid impermeable layer yet has high strength characteristics which permit high fluid-flow rates therethrough even at high pressures.